The present invention relates to torque and strain sensor, and more particularly this invention relates to a temperature insensitive fiber-optic torque and strain sensor.
Fiber-optic strain sensors have been developed using a wide variety of approaches, including fiber Bragg grating sensors [A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, xe2x80x9cFiber Grating Sensors,xe2x80x9d J. Lightwave Technol. 15(8), 1442-1463 (1997)], interferometric (Mach-Zehnder and Michelson) sensors [K. A. Murphy, W. V. Miller, T. A. Tran, A. M. Vengsarkar, and R. O. Claus, xe2x80x9cMiniaturized fiber-optic Michelson-type interferometric sensors,xe2x80x9d Appl. Opt. 30(34), 5063 5067 (1991)], white-light interferometers [S. C. Kaddu, S. F. Collins, and D. J. Booth, xe2x80x9cMultiplexed intrinsic optical fibre Fabry-Perot temperature and strain sensors addressed using white-light interferometry,xe2x80x9d Meas. Sci. Technol. 10, 416-420 (1999)], intrinsic polarimetric sensors based on polarization-maintaining (PM) fiber [W. J. Bock and W. Urbanczyk, xe2x80x9cTemperature desensitization of a fiber-optic pressure sensor by simultaneous measurement of pressure and temperature,xe2x80x9d Appl. Opt. 37(18), 3897-3901 (1998)], extrinsic polarimetric sensors [C. S. Sun, L. Wang, Y. Wang, and J. Lin, xe2x80x9cDesign of a high-sensitivity photoelastic optical fiber pressure sensor: a differential approach,xe2x80x9d IEEE Photon. Technol. Lett. 9(7) 976-978 (1997)], and extrinsic Fabry-Perot sensors [K. A. Murphy, M. F. Gunther, R. O. Claus, T. A. Tran, and M. S. Miller, xe2x80x9cOptical fiber sensors for measurement of strain and acoustic waves,xe2x80x9d Smart Sensing, Processing, and Instrumentation, Proc. SPIE Vol. 1918, 110-120 (1993)]. To varying degrees, all of these sensor types are plagued with the problem of cross-sensitivity to temperature. For example, polarimetric sensors that employ PM fiber exhibit a thermal apparent strain sensitivity on the order of 50 xcexcxcex5/xc2x0 C. [T. Valis, D. Hogg, R. M. Measures, xe2x80x9cThermal apparent-strain sensitivity of surface adhered, fiber-optic strain gauges,xe2x80x9d Appl. Opt. 31(34), 7178-7179 (1992)] and for other sensor types 10 xcexcxcex5/xc2x0 C. is typical [W. Jin, W. C. Michie, G. Thursby, M. Konstantaki, and B. Culshaw, xe2x80x9cSimultaneous measurement of strain and temperature: error analysis,xe2x80x9d Opt. Eng. 36(2), 598-608 (1997)].
U.S. Pat. No. 5,723,794 issued to Discenzo is directed to a photoelastic torque sensor that uses a photoelastic polymer detector in conjunction with a photoelastic image sensor (CCD camera) and a neural network. The photoelastic polymer sheet is bonded to the component being monitored. The CCD camera receives the phase shifted light signal from the photoelastic sheet and generates a electrical signals indicative of the phase shift produced in the beam reflected from the sensor sheet. The neural network then calculates the torque based on these signals.
U.S. Pat. No. 4,668,086 issued to Redner discloses a method and device for measuring stress and strain in a thin film by passing a multi-wavelength beam through the thin film and then splitting the transmitted signal into different spatially separated wavelength beams. The intensities of the beams at each wavelength are analysed to produce a measure of the strain in the film.
U.S. Pat. No. 4,123,158 issued to Reytblatt discloses a photoelastic strain gauge comprising a photoelastic polymer sheet coated on the opposing planar faces with reflective coatings. This produces a waveguide-like structure so that light is multiply reflected along the polymer sheet before it exits and this acts to produce an amplification of the visual patterns reflective of the strain produced in the photoelastic sheet from the underlying object.
U.S. Pat. No. 5,864,393 issued to Maris discloses an optical method for measuring strain in thin films that involves pumping the thin film with optical pump pulses and at different time delays applying optical probe pulses and detecting variations in the transient response to the probe pulses arising in part due to the propagation of a strain pulse in the film.
U.S. Pat. No. 5,817,945 issued to Morris et al. discloses a method of sensing strain using a photoluminescent polymer coating. The method is predicated on the relative changes or competition between radiative and non-radiative decay mechanisms of excited photoluminescent probe molecules in the coating in the presence and absence of strain in the coating. Regions of the coating under greater strain due to strain in the underlying substrate show up as brighter areas in the processed images.
U.S. Pat. Nos. 5,693,889 and 5,728,944 issued to Nadolink disclose a method of measuring surface stress and uses a wafer of single crystal silicon which must be embedded in the material being monitored so the silicon surface is even with the substrate surface. Fringe patterns in the light reflected off the silicon surface are indicative of the stress present at the surface.
U.S. Pat. No. 4,939,368 issued to Brown discloses an optical strain gauge comprising a diffraction grating applied to a surface and a light from a source having at least two frequencies is reflected off the surface and the phase differences between the beams at the two wavelengths is related to the strain in the surface.
U.S. Pat. No. 4,912,355 issued to Noel et al. is directed to a superlattice strain gauge using piezoelectric superlattice deposited onto the substrate being monitored. Strain in the underlying substrate will add internal strain present in the superlattice which significantly changes the optical properties among the different superlattice layers and these changes are monitored by the light probe.
U.S. Pat. No. 4,347,748 issued to Pierson discloses a torque transducer for measuring torque on a rotating shaft. The device is based on attaching optically flat mirrors to the shaft and reflecting a laser beam off each of the flats. The relative phase displacements of the beams is indicative of the torque on the shaft.
U.S. Pat. No. 5,298,964 issued to Nelson et al. discloses an optical stress sensing system that is based on directing three separate polarized light beams along three different optical axes in a single photoelastic sensing element. The applied stresses in the three directions are determined independently, and through the use of sum-difference techniques applied to the output signals, the results can be made insensitive to fluctuations in light source intensity and to losses in the optical fibers that deliver the light.
U.S. Pat. No. 4,777,358 issued to Nelson discloses an optical differential strain gauge in which a light beam traverses two photoelastic elements in series. The two are secured on opposite faces of the test specimen so that the specimen transfers tensile strain to one and shear strain to the other. A fiber optic polarization rotator is inserted in the optical path between the two elements so that the system measures the difference between the transferred tensile and shear strains and environmental effects common to the two elements cancel.
U.S. Pat. No. 4,556,791 issued to Spillman discloses a stress sensor in which a light beam passes sequentially through a voltage controlled wave plate and a photoelastic element that is bonded to a test specimen. The optical powers in the two polarizations at xc2x145 degrees to the applied stress axis are detected and the resulting voltages applied to a difference amplifier. The amplifier output is fed back to the wave plate to null out the net phase retardation. The feedback signal is used as measure of applied stress.
A great deal of research has been devoted to developing schemes to compensate for temperature dependence or to perform simultaneous measurements of both strain and temperature [8, J. D. Jones, xe2x80x9cReview of fibre sensor techniques for temperature-strain discrimination,xe2x80x9d in 12th International Conference on Optical Fiber Sensors, Vol. 16, 36-39, OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1997)]. Some very good results have been demonstrated, however reduced cross-sensitivity to temperature often comes at the cost of adding complexity to the strain sensing system.
It would be very advantageous to provide a combination strain and/or torque sensor having a reduced sensitivity to temperature.
It is an object of the present invention to provide a strain/torque sensor that has a low temperature sensitivity.
The present invention provides a simple design for a temperature-insensitive extrinsic polarimetric strain sensor. The sensing element is a thin sheet of photoelastic material that is bonded to the test object. It is illuminated with linearly polarized light with the polarization direction at 45 degrees relative to the strain-induced fast and slow axes in the photoelastic material. The sensor measures the difference between the strains along these two orthogonal directions. The reduced sensitivity of the sensor to temperature results from the fact that the illumination is perpendicular to the surface of the test object. All polarization components that are parallel to the surface will experience identical refractive index changes due to thermal effects. Consequently, a measurement of the difference in strains along two directions in the surface plane is expected to be insensitive to temperature.
In one aspect of the invention there is provided a method of measuring strain in a workpiece that is insensitive to ambient temperature fluctuations, comprising the steps of:
illuminating a strain-sensitive material with a beam of selectively polarized light, the beam of selectively polarized light being substantially perpendicular to the surface of the workpiece, the strain-sensitive material being in physical contact with the surface of the workpiece;
measuring an intensity of at least one polarization component of the beam of selectively polarized light making at least one pass through the strain-sensitive material, the at least one polarization component being tilted with respect to strain-induced fast and slow orthogonal axes in the strain-sensitive material so that it has a projection along both strain-induced fast and slow orthogonal axes; and
calculating from said intensity a difference between strains along the strain-induced fast and slow orthogonal axes in the strain-sensitive material, the difference being substantially independent of ambient temperature fluctuations.
In another aspect of the invention there is provided a method of measuring strain in a workpiece that is insensitive to ambient temperature fluctuations, comprising the steps of:
illuminating a strain-sensitive material with a beam of selectively polarized light, the beam of selectively polarized light being substantially perpendicular to the surface of the workpiece, the strain-sensitive material being in physical contact with the surface of the workpiece;
measuring a first intensity of a first polarization component and a second intensity of a second polarization component of the beam of selectively polarized light making at least one pass through the strain-sensitive material, the first and second polarization components being orthogonal to each other and tilted with respect to strain-induced fast and slow orthogonal axes in the strain-sensitive material parallel to the surface of the workpiece so that each of said first and second polarization components has a projection along both strain-induced fast and slow orthogonal axes; and
calculating from the first and second intensities a difference between strains along the strain-induced fast and slow orthogonal axes in the strain-sensitive material, the difference being substantially independent of ambient temperature fluctuations.
The present invention also provides a temperature insensitive strain sensor that is insensitive to ambient temperature fluctuations, comprising:
a strain-sensitive material adapted to be physically contacted to a surface of a workpiece;
light source means and light beam polarization means for illuminating said strain-sensitive material with a beam of selectively polarized light, said beam of selectively polarized light being substantially perpendicular to the surface of the workpiece; and
detection means for measuring an intensity of at least one of a first polarization component and a second polarization component in the beam of selectively polarized light making at least one pass through said strain-sensitive material, the at least one of a first and-second polarization components being tilted with respect to strain-induced fast and slow orthogonal axes in the strain-sensitive material parallel to the surface of the workpiece so that at least one of the at least first and second polarization components has a projection along both strain-induced fast and slow orthogonal axes; and
processing means connected to said detection means for calculating from the intensity of the at least one of a first polarization component and a second polarization component a difference between strains along the strain-induced fast and slow orthogonal axes in said strain-sensitive material, the difference between strains being substantially independent of ambient temperature fluctuations.